Call Admission Control

Call admission control (CAC) enables an access point to maintain controlled quality of service (QoS) when the wireless LAN is experiencing congestion. The Wi-Fi Multimedia (WMM) protocol deployed in CCXv3 ensures sufficient QoS as long as the wireless LAN is not congested. However, in order to maintain QoS under differing network loads, CAC in CCXv4 is required. Two types of CAC are available: bandwidth-based CAC and load-based CAC.

Bandwidth-Based CAC

Bandwidth-based, or static, CAC enables the client to specify how much bandwidth or shared medium time is required to accept a new call and in turn enables the access point to determine whether it is capable of accommodating this particular call. The access point rejects the call if necessary in order to maintain the maximum allowed number of calls with acceptable quality.

The QoS setting for a WLAN determines the level of bandwidth-based CAC support. To use bandwidth-based CAC with voice applications, the WLAN must be configured for Platinum QoS. To use bandwidth-based CAC with video applications, the WLAN must be configured for Gold QoS. Also, make sure that WMM is enabled for the WLAN. See the Information About Configuring 802.3 Bridging section for QoS and WMM configuration instructions.

Note

You must enable admission control (ACM) for CCXv4 clients that have WMM enabled. Otherwise, bandwidth-based CAC does not operate properly.

Load-Based CAC

Load-based CAC incorporates a measurement scheme that takes into account the bandwidth consumed by all traffic types (including that from clients), co-channel access point loads, and collocated channel interference, for voice applications. Load-based CAC also covers the additional bandwidth consumption resulting from PHY and channel impairment.

In load-based CAC, the access point continuously measures and updates the utilization of the RF channel (that is, the percentage of bandwidth that has been exhausted), channel interference, and the additional calls that the access point can admit. The access point admits a new call only if the channel has enough unused bandwidth to support that call. By doing so, load-based CAC prevents oversubscription of the channel and maintains QoS under all conditions of WLAN loading and interference.

Note

Load-based CAC is supported only on lightweight access points. If you disable load-based CAC, the access points start using bandwidth-based CAC.

Expedited Bandwidth Requests

The expedited bandwidth request feature enables CCXv5 clients to indicate the urgency of a WMM traffic specifications (TSPEC) request (for example, an e911 call) to the WLAN. When the controller receives this request, it attempts to facilitate the urgency of the call in any way possible without potentially altering the quality of other TSPEC calls that are in progress.

You can apply expedited bandwidth requests to both bandwidth-based and load-based CAC. Expedited bandwidth requests are disabled by default. When this feature is disabled, the controller ignores all expedited requests and processes TSPEC requests as normal TSPEC requests.

1 For bandwidth-based CAC, the voice call bandwidth usage is per access point and does not take into account co-channel access points. For load-based CAC, the voice call bandwidth usage is measured for the entire channel.

When video ACM is enabled, the controller rejects a video TSPEC if the non-MSDU size in the TSPEC is greater than 149 or the mean data rate is greater than 1 Kbps.

U-APSD

Unscheduled automatic power save delivery (U-APSD) is a QoS facility defined in IEEE 802.11e that extends the battery life of mobile clients. In addition to extending battery life, this feature reduces the latency of traffic flow delivered over the wireless media. Because U-APSD does not require the client to poll each individual packet buffered at the access point, it allows delivery of multiple downlink packets by sending a single uplink trigger packet. U-APSD is enabled automatically when WMM is enabled.

Traffic Stream Metrics

In a voice-over-wireless LAN (VoWLAN) deployment, traffic stream metrics (TSM) can be used to monitor voice-related metrics on the client-access point air interface. It reports both packet latency and packet loss. You can isolate poor voice quality issues by studying these reports.

The metrics consist of a collection of uplink (client side) and downlink (access point side) statistics between an access point and a client device that supports CCX v4 or later releases. If the client is not CCX v4 or CCXv5 compliant, only downlink statistics are captured. The client and access point measure these metrics. The access point also collects the measurements every 5 seconds, prepares 90-second reports, and then sends the reports to the controller. The controller organizes the uplink measurements on a client basis and the downlink measurements on an access point basis and maintains an hour’s worth of historical data. To store this data, the controller requires 32 MB of additional memory for uplink metrics and 4.8 MB for downlink metrics.

TSM can be configured through either the GUI or the CLI on a per radio-band basis (for example, all 802.11a radios). The controller saves the configuration in flash memory so that it persists across reboots. After an access point receives the configuration from the controller, it enables TSM on the specified radio band.

Note

Access points support TSM entries in both local and FlexConnect modes.

This table shows the upper limit for TSM entries in different controller series.

TSM Entries

5500

7500

MAX AP TSM entries

100

100

MAX Client TSM entries

250

250

MAX TSM entries

100*250=25000

100*250=25000

Note

Once the upper limit is reached, additional TSM entries cannot be stored and sent to Cisco Prime Infrastructure. If client TSM entries are full and AP TSM entries are available, then only the AP entries are stored, and vice versa. This leads to partial output. TSM cleanup occurs every one hour. Entries are removed only for those APs and clients that are not in the system.

Configuring Voice Parameters

Configuring Voice Parameters (GUI)

Step 1

Ensure that the WLAN is configured for WMM and the Platinum QoS level.

Step 2

Disable all WLANs with WMM enabled and click Apply.

Step 3

Choose Wireless and then Network under 802.11a/n or 802.11b/g/n, unselect the 802.11a (or 802.11b/g) Network Status check box, and click Apply to disable the radio network.

Select the Admission Control (ACM) check box to enable bandwidth-based CAC for this radio band. The default value is disabled.

Step 6

Select the Admission Control (ACM) you want to use by choosing from the following choices:

Load-based—To enable channel-based CAC. This is the default option.

Static—To enable radio-based CAC.

Step 7

In the Max RF Bandwidth text box, enter the percentage of the maximum bandwidth allocated to clients for voice applications on this radio band. Once the client reaches the value specified, the access point rejects new calls on this radio band.

The range is 5% to 85%. The sum of maximum bandwidth percentage of voice and video should not exceed 85%.

The default is 75%.

Step 8

In the Reserved Roaming Bandwidth text box, enter the percentage of maximum allocated bandwidth that is reserved for roaming voice clients. The controller reserves this bandwidth from the maximum allocated bandwidth for roaming voice clients.

To enable SIP CAC support, select the SIP CAC Support check box. By default, SIP CAC support is disabled.

Step 11

From the SIP Codec drop-down list, choose one of the following options to set the codec name. The default value is G.711. The options are as follows:

User Defined

G.711

G.729

Step 12

In the SIP Bandwidth (kbps) text box, enter the bandwidth in kilobits per second.

The possible range is 8 to 64.

The default value is 64.

Note

The SIP Bandwidth (kbps) text box is highlighted only when you select the SIP codec as User-Defined. If you choose the SIP codec as G.711, the SIP Bandwidth (kbps) text box is set to 64. If you choose the SIP codec as G.729, the SIP Bandwidth (kbps) text box is set to 8.

Step 13

In the SIP Voice Sample Interval (msecs) text box, enter the value for the sample interval.

Step 14

In the Maximum Calls text box, enter the maximum number of calls that can be made to this radio. The maximum call limit includes both direct and roaming-in calls. If the maximum call limit is reached, the new or roaming-in calls result in failure.

The possible range is 0 to 25.

The default value is 0, which indicates that there is no check for maximum call limit.

In the Video tab, select the Admission Control (ACM) check box to enable video CAC for this radio band. The default value is disabled.

Step 6

From the CAC Method drop-down list, choose between Static and Load Based methods.

The static CAC method is based on the radio and the load-based CAC method is based on the channel.

Note

For TSpec and SIP based CAC for video calls, only Static method is supported.

Step 7

In the Max RF Bandwidth text box, enter the percentage of the maximum bandwidth allocated to clients for video applications on this radio band. When the client reaches the value specified, the access point rejects new requests on this radio band.

The range is 5% to 85%. The sum of maximum bandwidth percentage of voice and video should not exceed 85%. The default is 0%.

Step 8

In the Reserved Roaming Bandwidth text box, enter the percentage of the maximum RF bandwidth that is reserved for roaming clients for video.

Step 9

Configure the SIP CAC Support by selecting or unselecting the SIP CAC Support check box.

SIP CAC is supported only if SIP Snooping is enabled.

Note

You cannot enable SIP CAC if you have selected the Load Based CAC method.

Step 10

Click Apply.

Step 11

Reenable all WMM WLANs and click Apply.

Step 12

Choose Network under 802.11a/n or 802.11b/g/n, select the 802.11a (or 802.11b/g) Network Status check box, and click Apply to reenable the radio network.

Step 13

Click Save Configuration.

Step 14

Repeat this procedure if you want to configure video parameters for another radio band.

Configuring Video Parameters (CLI)

Before You Begin

Ensure that you have configured SIP-based CAC.

Step 1

See all of the WLANs configured on the controller by entering this command:

show wlan summary

Step 2

Make sure that the WLAN that you are planning to modify is configured for WMM and the QoS level is set to Gold by entering this command:

show wlanwlan_id

Step 3

Disable all WLANs with WMM enabled prior to changing the video parameters by entering this command:

config wlan disablewlan_id

Step 4

Disable the radio network by entering this command:

config {802.11a | 802.11b} disablenetwork

Step 5

Save your settings by entering this command:

save config

Step 6

Enable or disable video CAC for the 802.11a or 802.11b/g network by entering this command:

config {802.11a | 802.11b} cac video acm {enable | disable}

Step 7

To configure the CAC method as either static or load-based, enter this command:

config {802.11a | 802.11b} cac video cac-method {static | load-based}

Step 8

Set the percentage of maximum bandwidth allocated to clients for video applications on the 802.11a or 802.11b/g network by entering this command:

config {802.11a | 802.11b} cac video max-bandwidthbandwidth

The bandwidth range is 5 to 85%, and the default value is 5%. However, the maximum RF bandwidth cannot exceed 85% for voice and video. Once the client reaches the value specified, the access point rejects new calls on this network.

Note

If this parameter is set to zero (0), the controller assumes that you do not want to do any bandwidth allocation and, therefore, allows all bandwidth requests.

Step 9

To configure the percentage of the maximum RF bandwidth that is reserved for roaming clients for video, enter this command:

config {802.11a | 802.11b} cac video roam-bandwidthbandwidth

Step 10

To configure the CAC parameters for SIP-based video calls, enter this command:

config {802.11a | 802.11b} cac video sip {enable | disable}

Step 11

Process or ignore the TSPEC inactivity timeout received from an access point by entering this command:

Click the Detail link for the desired access point to open the Clients > AP > Traffic Stream Metrics page.

This page shows the TSM statistics for this client and the access point to which it is associated. The statistics are shown in 90-second intervals. The timestamp text box shows the specific interval when the statistics were collected.

Step 5

See the TSM statistics for a particular access point and a particular client associated to this access point, as follows:

Hover your cursor over the blue drop-down arrow for the desired access point and choose 802.11aTSM or 802.11b/g TSM. The AP > Clients page appears.

Click the Detail link for the desired client to open the AP > Clients > Traffic Stream Metrics page.

This page shows the TSM statistics for this access point and a client associated to it. The statistics are shown in 90-second intervals. The timestamp text box shows the specific interval when the statistics were collected.

Viewing Voice and Video Settings (CLI)

Step 1

See the CAC configuration for the 802.11 network by
entering this command:

show ap stats {802.11a |
802.11b}

Step 2

See the CAC statistics for a
particular access point by entering this command:

In the example above, “MT” is
medium time, “Na” is the number of additional calls, and “exp bw” is expedited
bandwidth.

Note

Suppose an AP has to be
rebooted when a voice client associated with the AP is on an active call. After
the AP is rebooted, the client continues to maintain the call, and during the
time the AP is down, the database is not refreshed by the controller.
Therefore, we recommend that all active calls are ended before the AP is taken
down.

Step 3

See the U-APSD status for a particular
client by entering this command:

show client
detailclient_mac

Step 4

See the TSM statistics for a
particular client and the access point to which this client is associated by
entering this command:

show client tsm {802.11a |
802.11b}
client_mac
{ap_mac |
all}

The optional
all command shows all access points to which this
client has associated. Information similar to the following appears:

The statistics are shown in
90-second intervals. The timestamp text box shows the specific interval when
the statistics were collected.

Note

Clear the TSM statistics for
a particular access point or all the access points to which this client is
associated by entering this
clear client tsm {802.11a |
802.11b}
client_mac
{ap_mac |
all} command.

Step 5

See the TSM statistics for a
particular access point and a particular client associated to this access point
by entering this command:

show ap stats {802.11a |
802.11b}
ap_nametsm {client_mac |
all}

The optional
all command shows all clients associated to this
access point. Information similar to the following appears:

The statistics are shown in
90-second intervals. The timestamp text box shows the specific interval when
the statistics were collected.

Step 6

Enable or disable debugging
for call admission control (CAC) messages, events, or packets by entering this
command:

debug cac
{all |
event |
packet}{enable |
disable}

where
all configures debugging for all CAC messages,
event configures debugging for all CAC events, and
packet configures debugging for all CAC packets.

Step 7

Use the following command to
perform voice diagnostics and to view the debug messages between a maximum of
two 802.11 clients:

debug voice-diag
{enable |
disable}
mac-id mac-id2
[verbose]

The verbose mode is an
optional argument. When the verbose option is used, all debug messages are
displayed in the console. You can use this command to monitor a maximum of two
802.11 clients. If one of the clients is a non-WiFi client, only the 802.11
client is monitored for debug messages.

Note

It is implicitly assumed that
the clients being monitored are on call.

Note

The debug command
automatically stops after 60 minutes.

Step 8

Use the following commands to
view various voice-related parameters:

show client
voice-diag status

Displays information about
whether voice diagnostics is enabled or disabled. If enabled, will also
displays information about the clients in the watch list and the time remaining
for the diagnostics of the voice call.

If voice diagnostics is
disabled when the following commands are entered, a message indicating that
voice diagnostics is disabled appears.

show client
voice-diag tspec

Displays the TSPEC
information sent from the clients that are enabled for voice diagnostics.

show client
voice-diag qos-map

Displays information about
the QoS/DSCP mapping and packet statistics in each of the four queues: VO, VI,
BE, BK. The different DSCP values are also displayed.

show client
voice-diag avrg_rssi

Display the client’s RSSI
values in the last 5 seconds when voice diagnostics is enabled.

show client
voice-diag roam-history

Displays information about
the last three roaming calls. The output contains the timestamp, access point
associated with roaming, roaming reason, and if there is a roaming failure, the
reason for the roaming-failure.

show client
calls {active | rejected} {802.11a | 802.11bg | all}

This command lists the
details of active TSPEC and SIP calls on the controller.

Step 9

Use the following commands
to troubleshoot video debug messages and statistics:

In the Maximum Media Bandwidth (0-85%) text box, enter the percentage of the maximum bandwidth to be allocated for media applications on this radio band. Once the client reaches the specified value, the access point rejects new calls on this radio band.

The default value is 85%; valid values are from 0 to 85%.

Step 8

In the Client Phy Rate text box, enter the value for the rate in kilobits per second at which the client operates.

Step 9

In the Maximum Retry Percent (0-100%) text box, enter the percentage of the maximum retry. The default value is 80.

From the Max Streams per Radio drop-down list, choose the maximum number of allowed multicast direct streams per radio. Choose a value between 1 to 20 or No Limit. The default value is set to No Limit.

Step 12

From the Max Streams per Client drop-down list, choose the maximum number of allowed clients per radio. Choose a value between 1 to 20 or No Limit. The default value is set to No Limit.

Step 13

If you want to enable the best radio queue for this radio, select the Best Effort QoS Admission check box. The default value is disabled.

You can configure a controller to support calls from clients that do not support TSPEC-based calls. This feature is known as voice prioritization. These calls are given priority over other clients utilizing the voice pool. Voice prioritization is available only for SIP-based calls and not for TSPEC-based calls. If the bandwidth is available, it takes the normal flow and allocates the bandwidth to those calls.

You can configure up to six preferred call numbers. When a call comes to one of the configured preferred numbers, the controller does not check on the maximum call limit. It invokes the CAC to allocate bandwidth for the preferred call. The bandwidth allocation is 85 percent of the entire bandwidth pool, not just from the maximum configured voice pool. The bandwidth allocation is the same even for roaming calls.

Custom Voice—Enables custom voice EDCA parameters for 802.11a. The EDCA parameters under this option also match the 6.0 WMM EDCA parameters when this profile is applied.

Note

If you deploy video services, admission control (ACM) must be disabled.

Step 4

If you want to enable MAC optimization for voice, select the Enable Low Latency MAC check box. Otherwise, leave this check box unselected, which is the default value. This feature enhances voice performance by controlling packet retransmits and appropriately aging out voice packets on lightweight access points, which improves the number of voice calls serviced per access point.

Note

We do not recommend you to enable low latency MAC.
You should enable low latency MAC only if the WLAN allows WMM clients.
If WMM is enabled, then low latency MAC can be used with any of the EDCA
profiles.

Step 5

Click Apply to commit your changes.

Step 6

To reenable the radio network, choose Network under 802.11a/n or 802.11b/g/n, select the 802.11a (or 802.11b/g) Network Status check box, and click Apply.

This feature enhances voice performance by controlling packet retransmits and appropriately aging out voice packets on lightweight access points, which improves the number of voice calls serviced per access point. The default value is disabled.